Alumina, has many desirable intrinsic physical properties including mechanical strength, temperature resistance, and electrical resistance, which are primarily determined by the crystal structure. Most processes used to obtain functional ceramic materials, such as alumina, consist of sintering (at a sufficiently high temperature) compressed particulate-porous compacts of fine, crystalline grains of appropriate starting materials to form strong polycrystalline products. During sintering, the particulate-porous compact undergoes changes in its structure which are common to porous fine-grained crystalline materials. There is an increase in grain size, there is a change in pore shape, and there is change in pore size and number. Sintering usually produces a decrease in porosity and results in densification of the particulate compact. The sintering process can be adjusted to control final grain size and density.
Reactions and thermal transformations that take place during sintering result in structures composed of an assembly of crystalline grains having irregular shapes and different orientations (polycrystalline). Depending on the starting materials and thermal treatment, more than one phase can develop during sintering. Polycrystalline structures with more than one phase are commonly called multiphase materials. The final properties of a polycrystalline structure are dependent on the interfaces or grain boundaries, and the presence or absence of multiphase material.
Additives may be used in the sintering of ceramic alumina compositions for several purposes. These purposes may include: grain growth repression or acceleration, reduction in sintering temperature, alteration in porosity and removal of impurities. In the preparation of alumina, if MgO is used as an additive, or if sufficient Mg impurity is present, spinel (MgAl.sub.2 O.sub.4) can form between grains and at triple points. (See Haroun, N. A. and Budworth, D. W., "Effects of Additions of MgO, ZnO and NiO on Grain Growth in Dense Alumina" Transactions of the British Ceramic Society, 69 (1970) 73-79. ) This is especially so when the amount of Mg in the alumina exceeds the solubility limit at the sintering temperature. Coble, R. L., U.S. Pat. No. 3,026,210 discloses a composition and method of preparation of transparent alumina, wherein the alumina is doped with up to 0.5 weight percent magnesia present primarily as an alumina-magnesia spinel.
The majority of commercially available aluminas are deliberately sintered via a liquid phase route through the use of additives such as oxides of silicon, calcium, sodium and potassium, often added in the form of minerals or clays. These additives enhance formation of silicate liquid phases, and the presence of these liquid phases during sintering aid densification at relatively low firing temperatures. They also form glassy (silaceous) grain boundary films on cooling which are readily attacked by aqueous HF acid. This attack of grain boundary films results in a rapid in-service disintegration of liquid phase sintered polycrystalline alumina components.
Even when liquid-forming additives are not used, sufficient impurities are generally present in the starting alumina powder to result in trace liquid formation upon sintering. Typical impurities include SiO.sub.2, CaO, Fe.sub.2 O.sub.3, TiO2, K.sub.2 O and MgO.
Additionally, Genthe, W. et al., "Influence of Chemical Composition on Corrosion of Alumina in Acids and Caustic Solutions", J. Eur. Ceramic Society 9 (1992) 417-425, specifically addresses the corrosion resistance of alumina doped with MgO and concludes that alumina samples doped with less than approximately 11,500 ppm Mg.sup.2+ to Al.sup.3+ ions, do not demonstrate any appreciable resistance to HF.
Improved materials of construction which can withstand acid, especially hydrofluoric acid (HE), exposure are needed in industry. The choice of materials for HF environments is limited as HF (especially aqueous HF) is corrosive to most metallic and many non-metallic materials. This narrows the choice of materials of construction for articles such as reaction vessels and handling equipment. The material choice is further limited as process temperatures increase, since corrosion is typically a reaction requiring energy to activate. Polytetrafluoroethylene (PTFE) is often used in HF-containing environments, which limits processing temperatures to about 100.degree. C. Therefore, the extension of existing processing temperatures is an important additional factor influencing the synthesis of new materials for use in HF environments. The principal advantage offered by ceramics in these environments is the ability to increase process temperatures with attendant improvements in process efficiency.